This invention relates to a method of labelling an article, and to a method of authentication for an article. It also relates to an identification means or label for use in such a method.
Magnetic tape having a permanent pattern of a detectable magnetic quantity is known from GB A 1331604, which is incorporated herein by reference, and GB A 2309568. Such tape is available from Thorn Secure Science Limited under the UK registered trade mark xe2x80x9cWATERMARKxe2x80x9d tape, and is used as an identification or authentication means on articles of value such as bank cards or credit cards.
At the present state of the art the maximum data packing densities used in volume manufacture of this tape is 39 bits per inch (1.53 bits per mm) using industry standard F2F coding. The data format used in Watermark tape requires marker portions, known as xe2x80x9csentinelsxe2x80x9d, with a binary digit string between successive sentinels. Each sentinel comprises 10 bits, with the data between sentinels typically comprising 60 bits (12 characters each comprising 5 bits). The data between sentinels may be incrementing or non-incrementing. If the data is non-incrementing, then the data stored on the tape is periodic, having a period of 70 binary digits. At a data packing density of 1.3 bits per mm, this implies that over 53 mm of tape must be applied to an article of value in order to identify it correctly, and if part of the tape gets damaged, or the read head bounces at the edge of the label then there is no redundancy, and so the data cannot be read correctly. Thus Watermark tape is not particularly convenient for use on small articles or documents of value.
One way to reduce the pitch of data on such tape would be to decrease the length of the sentinel. In GB-A-2 021 835 a 5 bit sentinel and eight 5 bit characters was suggested. However, such a scheme has been found not to be practical because of significant interaction between the sentinel and the data, resulting in unacceptable read errors. Another drawback with this scheme is that the data can only be read in the forward direction and not in the reverse direction. To overcome this problem a sentinel having 10 bits has been employed for many years. This normally includes a long string of binary 1""s, such as 00111 11101 or 10111 11000. When combined with 5 bit data characters which use 4 bits to represent decimal characters 0 to 9 and the 5th bit for xe2x80x9coddxe2x80x9d parity this makes sure that the sentinel cannot occur in the twelve 5 bit data characters.
According to a first aspect of the invention, there is provided a method of labelling an article as claimed in claims 1-12. According to a second aspect of the invention there is provided an article so labelled as claimed in claim 15.
According to a third aspect of the invention, there is provided an identification means for labelling an article as claimed in claims 16-18.
According to a fourth aspect of the present invention there is provided a method of identifying an article as claimed in claims 13 and 14.